tracing: Fix documentation on tp_printk cmdline option

kernel-parameters.txt incorrectly states that workings of
kernel.tracepoint_printk sysctl depends on "tracepoint_printk kernel
cmdline option", this is a bit misleading for new users since the actual
cmdline option name is tp_printk.

Fixes: 0daa2302 ("tracing: Add tp_printk cmdline to have tracepoints go to printk()")
Signed-off-by: Vitaly Chikunov <vt@altlinux.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240323231704.1217926-1-vt@altlinux.org